MFCs are well established, being bio-electrochemical transducers that convert biochemical energy to electrical energy. They convert the chemical energy of organic feedstock into electricity using micro-organisms, which act as biocatalysts. The basic principle of operation, i.e. the extraction of electrons from an aqueous fuel source and their transfer onto electrode surfaces, has certain similarities to a conventional fuel cell.
MFCs commonly comprise a body, an anodic chamber, an anode, a cathode, a proton exchange membrane (PEM) window, an anodic fluid input and an anodic fluid output. The MFCs can be fed in a variety of ways, from manual periodic or automatic periodic (using simple mechanical valves), to a continuous flow into and between individual MFCs in an array. Generally, the anodic chamber is sealed to the outside with the exception of the PEM window, a conducting wire to the anode, which is sealed through the body of the vessel, and the anodic input/output tubes, which are sealed to the outside world but do connect the MFC units together.
MFCs represent a promising technology for sustainable energy production and waste treatment. They can extract energy from a fuel source such as wastewater by exploiting microbial communities in the anodic compartment that metabolise organic components in the feedstock. Electrons generated from these reactions travel through an externally connected circuit, from the anode to the cathode, thus producing a charge or current flow. Protons and electrons combine at the cathode, reducing oxygen to water. There are a number of limiting factors that influence energy generation and performance of a MFC. These include the rates of fuel oxidation and electron transfer to the anode by the microbes, the migration of protons to the cathode through the PEM and the oxygen supply and reduction reaction at the cathode.
Most existing systems involve either a single MFC with multiple anodes and cathodes, or a stack of MFCs that are securely and mechanically joined together, but that are not fluidically isolated, such that individual MFCs cannot be removed from the stack. This may be inefficient, as different fuel sources may need different lengths of time or bacterial communities to ensure that the feedstock is completely broken down. A degree of flexibility in the arrangement of the MFCs or electrodes is therefore favourable.
The water industry is energy intensive and consumes about 3% of the total energy used in the UK. The industry is responsible for approximately four million tonnes of greenhouse gas emission (CO2 equivalent) every year and although this only accounts for less than 1% of total UK emissions, the amount is rising year on year. Water and energy management are inter-related issues; the energy required to treat wastewater is high and the ongoing tightening of water quality standards will lead to increases in energy usage. The local and global environment would benefit from reduced energy usage and increased water quality. There is a clear un-met need for technologies that can reduce energy usage during wastewater treatment.
As noted above, one of the major applications of MFCs is in the clean up of waste, such as that derived from bioenergy generation, compost, municipal waste, food and biological waste, landfill leachate, and wastewater. Another potential application is power generation for low-power requiring systems, electronics, laptops, LEDs, small sensors, microprocessors, wristwatches, clocks, calculators, DC motors that drive fans, wheels of toys/robots, charging small devices such as mobile phones and i-Pods, charging larger devices such as laptops, DC-operated refrigerators/freezers and USB powered microscopes. Alternatively, MFCs could be used to recharge the internal batteries of said devices.
As well as being environmentally friendly, MFCs have low manufacturing costs and the production of energy by MFCs has the potential to be continuous over months or years. Current MFCs provide high substrate to electricity conversion efficiencies, but have low energy transformation rates (Ieropoulos et al., 2008). The typical sustainable voltage output from a MFC with a 25 millilitre anodic chamber with an oxygen-diffusion cathode and plain carbon veil electrodes is of the order of 0.5 V (open circuit). Higher open-circuit values of 1 V have been reported from individual MFCs under special conditions, which are closest to the theoretical maximum of 1.14 V. Thus, in order to produce sufficient voltage (1.5 V) and/or power to reside within the operating range of silicon-based circuitry, it is necessary to either scale up one single unit (but this cannot increase voltage) or to connect multiple units together.
As the maximum electrical power output of a MFC is directly proportional to the substrate utilisation rate of the organic material in the feedstock, all the mechanisms to increase power output of a stack of MFCs are automatically transferable to the rate and efficiency of the system for transforming organic waste into less harmful liquid outputs. The two potential applications listed above are therefore closely linked, and improving the efficiency of one application is likely to improve that of the other.
The electrodes in a MFC can be made of a variety of materials. The electrodes must have a high conductivity and a high surface area for bacterial colonisation, as well as high porosity to reduce diffusion limitations. For these reasons, carbon-fibre veils are often used.
PEMs can also be made from a variety of materials. Different materials have different properties, with some being more susceptible to polarity reversal, and others being better at preventing the flow of certain cations. This can be important for certain applications in which only hydrogen ions are needed to reach the cathode, though this is not as applicable for wastewater treatment.
The materials used to make the body of the MFC itself can vary depending on the application, but plastics, ceramics, soft polymers and coated metal alloys are often used. Plastics materials will be mainly of the thermo-plastics category, as these are cheap and rapidly manufactured on a large scale.
As mentioned above, it has been shown that a means to optimise power output (and thus ‘waste’ utilisation) is via multiple smaller MFCs that are connected together, rather than larger individual cells (Ieropoulos et al., 2008). However, there is still debate within the field on this point, with some believing that larger cells, that may also include multiple electrodes in a single cell, are better (Jiang et al., 2011 and Scott et al., 2007). Connecting multiple smaller MFCs has the advantage that the number of smaller MFCs can be changed with changing requirements. In order to increase power output in existing MFC stacks, cells have been connected in series, with a continuous flow of fluid feedstock running between each cell.
There are many examples of arrangements of multiple MFCs connected in series (see for example WO2010/049936, US2007/0048577 and US2010/0003543). However, in the arrangements disclosed in these documents, the MFCs are in liquid communication and so are fluidically connected. It has been shown that when MFCs are fluidically connected, the maximum power cannot be obtained. Ieropoulos et al., 2008 demonstrated that the voltage was 3-fold higher in isolated MFC stacks than in fluid-linked stacks. This difference may be due to high shunt losses that are incurred by a “short-circuit” phenomenon. This phenomenon occurs when the MFCs are electrically connected in series (to step-up the voltage), but the fluidic link joining the MFCs together is opposing that, bringing the units into the equivalent of a parallel connection. This results in a lower than expected voltage output. The shunt losses may be reduced by connecting the cells in a series/parallel manner, but they are not eliminated. The losses are even greater when the fluid medium has high levels of salt electrolytes.
Fluid connectivity can also lead to polarity reversal, which in turn reduces the power output of the cell (Ieropoulos et al., 2010). Polarity reversal in a stack is caused by the fact that defective MFC units develop a higher internal resistance than the other MFCs in the stack. This can be the result of starvation, heavy loading, but more often it is the result of a fouled membrane. In this situation, the solution in the anodic chamber becomes less negatively charged. If the load connected between the anode and the cathode is of sufficiently low resistance to allow the flow of the now scarce electrons at very low rates, this further reduces the negativity of the anode. This has a detrimental effect on the power output of the arrangement.
Copper plating of the cathode in a MFC has been shown to increase the conductivity of carbon electrodes, as well as having other beneficial effects. US 2010/0151279 discloses the use of copper plated cathodes in MFCs. However, this patent publication relates to a single MFC, and does not mention or suggest the possible effects of copper when multiple MFCs are connected together. Copper is also only mentioned as one of the many potential coating materials to enhance a desired reaction at the cathode. US2010/0151279 provides no evidence that copper was tested as a coating material, or that it increases power output.